One particular type of electrical circuit device that has a porcelain enameled steel or other glass or ceramic coated metal substrate is produced by punching out metal stampings in various shapes from sheet metal of a desired thicknesses. After a metal pretreatment, these metal stampings are electrophoretically dipcoated in a slurry of particles formed of porcelain enamel or other dielectric or resistive materials. Then the coated product is fired (i.e. heated to a sintering or fusing temperature) for a period of time sufficient to sinter or fuse the coating or deposit and create a bond between the sintered layer and the metal surface of the substrate.
The resulting coated metal substrate can be used for circuit boards, thermal sinks, thermal barriers, RF shielding, magnetic flux conduction, mechanical attachments and other related applications. Such coated metal substrates that are coated with a porcelain enamel material are commonly referred to as porcelain enameled metal substrates (PEMS) or ceramic coated metal substrates (CCMS).
Discrete metal foils can be attached to the resulting ceramic coated metal substrates by means of conventional techniques, such as soldering, adhesive attachment, etc. The metal foil patterns can serve as switch contacts, circuit traces to carry current, and terminals for interconnection of external wires.
In products of this type, it is often desirable to create a void or via in the sintered coating so that a portion of the conductive pattern formed on the dielectric or electrically resistive surface can be electrically connected to the base metal which is typically a conductive metal such as steel. These voids or access areas can be designed in any desired configuration although it has been determined that the area is desirably less than half of the total surface area of the substrate.
In the prior art, the creation of voids in the dielectric or resistive surface coating has been difficult and expensive. One method of forming a void is to abrade or wear away the dielectric material from the underlying base metal. This abrading process generates considerable dust and potentially exposes workers to a hazardous environment thereby requiring that certain safeguards be employed. Normally, a special mask is required, and following the operation, quantities of dust and dirt must be removed from the work area. Also, a secondary cleaning operation is generally required. This process alone does not prepare the surface of the metal substrate to receive a conductive layer such as a thick film. Additional steps must be taken in order to achieve a mechanical bond between a thick film and the resulting exposed surface portion of the substrate. This approach to creating voids or access to the base metal is quite costly and not suitable for creating relatively large area voids.
A related problem with prior art circuit devices having porcelain enameled steel or other glass or ceramic coated metal substrates, relates to the application of a conductive pattern to the dielectric or resistive surface. For example, when a thick film-type conductive layer is applied, the film may serve some purposes adequately, such as in the case of a low current rheostat, however, the thick film conductor fails frequently when subjected to repeated high current on/off (or open circuit, closed circuit) conditions. The failure results from the cycling of the switching device under relatively high current conditions.
One solution to this problem has been to apply a shaped stainless steel or copper foil to the surface of a thick film conductor pattern. This may be done, for example, by soldering and achieves what might be termed a hybrid conductor bus. One application of this device, for example is a switch, wherein the foil surface is contacted by one or more spring loaded contact fingers on a rotating rheostat. The coated substrate may be located inside a rheostat housing and the mechanical detent of the switch housing may be used to achieve on or off circuit conditions. One of the contactors wipes across or maintains contact with the hybrid conductor bus at all times while another contactor, depending upon the position of the coated substrate, wipes across the dielectric or resistive surface portion or another metal foil portion of the hybrid conductor bus.
While the soldered metal foils on the thick film have been capable of handling the current level involved, they have presented other difficulties. For example, the tactile response or "feel" perceived by the human operator as the substrate is rotated through the on/off positions of the rheostat housing has an irregular or "bumpy" feeling.
Another problem occurs when a contact finger instantaneously connects to a stainless steel hybrid bus. Due to the high current being switched at the point of initial contact, a small arc occurs between the contact finger and the stainless steel foil. This arc may cause the contact finger and the stainless steel foil to weld together momentarily. These problems often result from the difference in elevation between the dielectric coated substrate surface and the contact surface of the metal foil. Also, they occur as a result of the weld characteristics of the materials used in both the stainless steel foil and the spring-loaded contact finger.
The methods and resulting devices of the present invention resolve the difficulties described above and afford other features and advantages heretofore not obtainable.